This invention relates to integrated circuit packages and methods for fabricating such packages.
One common integrated circuit package of the prior art is comprised of a plurality of thin flat ceramic layers which are laminated together and which have a cavity that holds an integrated circuit chip. This cavity in cross section has staircase-shaped sidewalls. Patterned electrical conductors lie on the surfaces of the ceramic layers; and, on the steps of the cavity, these conductors are shaped as wire bonding pads. Discrete wires are bonded between these bonding pads and the integrated circuit chip to carry signals to and from the chip.
Since the cavity is staircase-shaped, several rows of bonding pads lie around the chip. This is desirable because it enables a large number of signals to be sent to and from the chip. A trend in integrated circuits is to increase the number of these signals, and today chips are fabricated with over 100 bonding pads.
A problem, however, with the above described integrated circuit package is that it is expensive to manufacture. One factor in this expense is the cost of a furnace which is needed to cure the ceramic at temperatures of about 2800.degree. F. Also, the furnace must be operated 24 hours a day to keep its temperature stable.
Another factor in this expense is low yield. During the curing process, the ceramic shrinks at rates which are very difficult to control and this makes some packages unusable. Also during the curing process, blisters can occur between some of the ceramic layers in the packages; and some of the conductors may not properly adhere to the ceramic layers. These problems combine typically lower the yield of ceramic packages to only 5%-50%.
Accordingly, efforts have been made to develop a lower cost package; and to that end, epoxy-glass has been used in place of ceramic. This change of materials eliminates all of the above described furnace problems, shrinkage problems, blister problems, and conductor adhesion problems. However, with epoxy-glass packages, adhesive layers are needed between the epoxy-glass layers in order to laminate and permanently hold the epoxy-glass layers together. No such adhesive is needed in ceramic packages, and this adhesive causes new problems.
Specifically, during lamination of the epoxy-glass layers, the adhesive softens and flows; and if the adhesive flows over the bonding pads on the steps of the staircase-shaped cavity, the package becomes unusable. A cured adhesive on the bonding pads cannot be removed without destroying the pads.
In the prior art, no method was provided for stopping such a flow of the adhesive. Consequently, epoxy-glass packages were not made with staircase-shaped sidewalls. Instead, the cavity merely had vertical sidewalls and the bonding pads were placed on only the top epoxy-glass layer. This, however, has a serious drawback in that it severely limits the number of signals that can be sent to and received from the integrated circuit chip.
Accordingly, a primary object of the invention is to provide an improved integrated circuit package and method of fabricating same which overcomes all of the above prior art problems.